FIG. 12 shows a related-art type of optical transmitter. As shown, the optical transmitter is composed of a flip-flop circuit 106, a modulator 109, a Laser diode (hereinafter LD), a current source 101, a photodiode (PD), and an automatic power control circuit 102.
The flip-flop circuit 106 captures a data signal DT in synchronization with a clock signal CL to output light-on/off signals (of positive phase and negative phase). A transistor Q1 and a transistor Q2 of the modulator 109 are supplied at the bases thereof with the positive-phase light-on/off signal and the negative-phase light-on/off signal to perform a differential operation. When the transistor Q2 is turned on, the laser diode (LD) is supplied with a drive current from the current source 101 to generate an optical pulse signal.
The photodiode (PD) converts part of the optical signal outputted from the Laser diode (LD) into an electrical signal. In order to set the amplitude of this electrical signal to a predetermined level, the automatic power control circuit 102 adjusts the magnitude of the current coming from the current source 101. Consequently, the optical output power of the Laser diode (LD) is maintained at a constant level.
Details of the above-mentioned optical transmitter are disclosed in Japanese Laid-open Patent No. Hei 6-97889, for example. In an optical transmission system for use in a public communication network, a station-side communication device is connected to plural subscriber-side communication devices by fiber optics. Some such optical transmission systems are based on PDS technology in which the optical fiber of the station-side communication device is coupled with each of the optical fibers of the plural subscriber-side communication devices through a passive optical divider/coupler such as a star coupler. Also, in the above-mentioned optical transmission system, while communication is being performed between the station-side communication device and one of the subscriber-side communication devices, another subscriber-side device may be powered on/off.
However, in the communication device having the above-mentioned related-art optical transmitter, the circuit operation in the communication device may be made unstable by the power on/off operation, causing the Laser diode (LD) to emit the light erroneously. This erroneous light emission is caused when the source voltage drops more than the threshold, thereby causing the flip-flop circuit 106 to output an error light-on signal to the modulator 109 by way of example. Consequently, if the communication device having the above-mentioned related-art type of optical transmitter is used in an optical transmission system based on PDS technology, the optical signal caused by erroneous light emission at a power on/off operation, affects the optical fibers of other communication devices through the optical divider/coupler, thereby interfering with the communication of these devices.